diff --git a/stm-containers.cabal b/stm-containers.cabal
--- a/stm-containers.cabal
+++ b/stm-containers.cabal
@@ -1,7 +1,7 @@
 name:
   stm-containers
 version:
-  0.2.0
+  0.2.1
 synopsis:
   Containers for STM
 description:
@@ -97,6 +97,7 @@
     -- control:
     list-t >= 0.2.4 && < 0.3,
     focus >= 0.1.2 && < 0.2,
+    transformers >= 0.3 && < 0.5,
     -- debugging:
     loch-th == 0.2.*,
     placeholders == 0.1.*,